1. Field of the Invention
The present invention relates generally to an automotive vehicle and more specifically to a flow control arrangement via which the degree power steering assist is reduced with an increase of vehicle speed and which obviates sudden changes in assist due to changes in load applied to ancillary equipment such as an alternator or the like.
2. Description of the Prior Art
In order to simplify engine arrangements it is a common practice to drive ancillary or auxiliary apparatus such as a power steering pump and alternator using the same belt and pulley arrangement. Further, in order to both protect the alternator from being rotated at excessively high rotational speeds and to improve engine fuel economy, a step down transmission is often disposed between the engine and drive pulley via which the pump and alternator are driven. With this type of transmission if, as shown in FIG. 1, the load on the alternator is light, then the transmission is arranged to downshift at an engine speed of 1,200 RPM, for example, while if the load on the alternator is high, due to the operation of an air conditioner unit or the like, then the downshift is made at a higher engine speed (for example, 2,150 RPM) so as to ensure that the output of the alternator is adequate to meet the demand. An example of the above mentioned transmission may be found in Japanese Utility model application first publication 57-67148.
It is also a current practice to vary the output of the power steering pump in a manner to provide a high degree of power steering assist at low engine speeds when the steering is heavy and to reduce the assist as the vehicle speed increases in order to give the steering a solid or stable "feel" and prevent dangerous steering phenomenon due to accidental steering wheel movement at high vehicle speeds.
A valve which provides such a reduction in power steering assist is shown in FIGS. 2 and 3 of the drawings. This valve takes the form of a so called "flow-down" valve which, due to its construction, induces a reduction in output to the power steering control valve and servo, upon the displacement of the pump per se, reaching a given level.
FIG. 4 shows, in graphical form, the performance characteristics provided by the above mentioned "flow-down" valve. As shown, upon the pump speed reaching 2,125 RPM the flow-down valve induces a gradual reduction in output. This reduction terminates upon the pump speed reaching a value of approximately 3700 RPM, whereafter the output becomes essentially constant. Accordingly, the power assist at engine speeds up to 1,700 RPM is high (see section B) enabling easy vehicle maneuvering during parking etc., and then gradually decreases (see section C) to a level (D) suitable for medium-high speed cruising.
In more detail, this flow-down valve includes a body 10 which, in this case, is integral with the body of the pump 12 and therefore essentially part of the pump. The body 10 is formed with bore 14 in which a spool 16 is reciprocally disposed. The body further includes a port 15 via which the discharge from the pump 12 is directly introduced, a drain port 18 and a discharge port 20 which fluidly communicates with a control valve 22 and a power steering servo 24. The spool 16 is received in the bore so as to define a feed back chamber 26. A spring 28 is disposed in this chamber 26 and arranged to urge the spool 16 in a direction which tends to close the drain port 18 and to close an orifice 30 which provides fluid communication between a pressure chamber 32 and the discharge port 20. As shown, the port 15 opens into the pressure chamber 32. A first by-pass passage 34 leads from the pressure chamber 32 to an orifice 36. A second by-pass passage 38 leads from a chamber 40 defined between lands 41, 42 of the spool 16 and the feedback chamber 26 to the discharge port 20. The orifice 36 also communicates with this chamber.
With this arrangement, upon the pressure in the pressure chamber 32 increasing, the spool 16 is forced back against the force of the spring 28 and the decreasing pressure in the feedback chamber 26 in a manner to increase the communication between the pressure chamber 32 and the drain port 18 while simultaneously modifying the communication between the by-pass passage 34 and the chamber 40 via the partial or total closure of the orifice 36 by land 41.
However, this type of valve, when used in combination with the previously mentioned transmission arrangement encounters the drawback that, as shown in FIG. 5, should the transmission effect a downshift while the valve is effecting a flow reduction or flow-down, then as the rotational speed of the pump 12 drops due to the downshift, the pressure in the pressure chamber 32 drops and the spool 16 moves to the left as shown in the drawings. This tends to close off the drain port 18 and cause a sudden sharp increase in the amount of fluid fed to the power steering control valve 22 and servo 24. This of course suddenly increases the power steering assist at a time when a gradual decrease is being effected and thus tends to impart a disconcerting feel to the vehicle steering.
One way of avoiding this problem is to set the higher of the two downshift speeds at either 1,700 or 3,100 RPM. Viz, on either side of the engine speed range in which the flow reduction is produced by the flow-down valve (see section C in FIG. 4). However, while avoiding the variation in steering assist, this induces the problem that if the high speed downshift is made at 1,700 RPM an expensive alternator having a particularly high capacity is required (because of the relatively low rotational speed), while if made at 3,100 RPM, the alternator tends to be rotated at excessive speeds and induces increased engine fuel consumption.